Carbon nanotubes (or CNTs) possess particular crystalline structures, of tubular and hollow shape, formed by rolling up one or more individual graphite sheets. In the case of nanotubes comprising several roiled-up sheets or walls, the rolling-up operation is coaxial along a longitudinal direction. A distinction is thus made between single-walled nanotubes (or SWNTs) and multiwalled nanotubes (or MWNTs).
CNTs may be prepared by known methods. There are several processes for synthesizing CNTs, especially electrical discharge, laser ablation and chemical vapour deposition (CVD) which enables large quantities of carbon nanotubes to be manufactured, and therefore obtained for a manufacturing cost compatible with their bulk use. The CVD process specifically consists in injecting a carbon source at relatively high temperature onto a catalyst, which may itself consist of a metal such as iron, cobalt, nickel or molybdenum, which is supported on an inorganic solid such as alumina, silica or magnesia. The carbon sources may include methane, ethane, ethylene, acetylene, ethanol, methanol or even a mixture of carbon monoxide and hydrogen (the HIPCO process).
CNTs are produced, for example, by Arkema, Nanocyl, Iljin and Showa Denko.
Graphenes are isolated and individualized sheets of graphite, but very often assemblies comprising between one and a few tens of sheets are referred to as graphenes. Unlike carbon nanotubes, they have a more or less planar structure, with corrugations due to thermal agitation that are even greater when the number of sheets is reduced. A distinction is made between FLGs (Few Layer Graphenes), NGPs (Nanosized Graphene Plates), CNSs (Carbon NanoSheets) and GNRs (Graphene NanoRibbons).
Various processes for preparing graphenes have been proposed, including that of A. K. Geim of Manchester, which consists in peeling, in successive layers, graphite sheets by means of an adhesive tape (“Scotch tape” method), Geim, A. K., Science (2004), 306, 666.
It is also possible to obtain particles of graphenes by cutting carbon nanotubes along the longitudinal axis (“Micro-Wave Synthesis of Large Few-Layer Graphene Sheets in Aqueous Solution of Ammonia”, Janowska, I. et al., NanoResearch, 2009 or “Narrow Graphene nanoribbons from Carbon Nanotubes”, Jiao, L. et al., Nature. vol. 458, p. 877-880, 2009. Other processes have been widely described in the literature.
Graphenes are produced, for example, by Vorbeck Materials and Angstron Materials.
From a mechanical standpoint, the CNTs exhibit excellent stiffness (measured by Young's modulus), comparable to that of steel, while at the same time being extremely light. Furthermore, they exhibit excellent electrical and thermal conductivity properties making it possible to envisage using them as additives in order to confer these properties on various materials, especially macromolecular materials, such as thermoplastic or elastomeric polymer materials.
Various approaches have been envisaged up till now for dispersing moderate amounts of CNTs in polymer matrices, for the purpose in particular of improving their electrostatic dissipation capability without affecting their mechanical properties, and thus of allowing the manufacture, from said matrices, of electronic components or coating panels, for example for the motor vehicle industry.
Furthermore, from the industrial standpoint, it is desirable to provide composites highly filled with CNTs and capable of being diluted to the desired concentration in various polymer matrices.
Unfortunately, CNTs prove to be difficult to handle and disperse, because of their small size, their pulverulence and possibly, when they are obtained by the CVD technique, their entangled structure which moreover generates strong van der Waals interactions between their molecules.
The poor dispersibility of the CNTs significantly affects the performances of the composites that they form with the polymer matrices into which they are introduced. In particular, the appearance of nanocracks, that are formed in the nanotube aggregates, is observed which results in an embrittlement of the composite. Furthermore, as far as the CNTs are poorly dispersed, it is necessary to increase their content in order to attain a given electrical and/or thermal conductivity.
The poor dispersibility of the carbon nanotubes is especially observed in the case of thermoplastic and/or elastomeric polymer matrices, in particular when the polymer is used in the form of granules, as described, in particular, in document US 2004/026581.
In order to overcome these drawbacks, various solutions have already been proposed in the prior art.
One solution, described in document WO 09/047,466 by the Applicant, consists in preparing a masterbatch from carbon nanotubes in powder form and from a thermoplastic and/or elastomeric polymer in powder form, the masterbatch being itself in a solid agglomerated form such as a granule; next, this masterbatch may be introduced into a thermoplastic and/or elastomeric polymer composition.
Another solution consists in producing a CNT dispersion in a solvent and a monomer and in carrying out an in situ polymerization resulting in the formation of functionalized CNTs. This solution is however complex and may prove to be expensive depending on the products used. Moreover, the grafting operations run the risk of damaging the structure of the nanotubes and, as a consequence, their electrical and/or mechanical properties.
Furthermore, attempts have been made to mix CNTs with a thermoplastic polymer matrix in a compounding tool conventionally used for obtaining composites based on thermoplastic polymers. However, it has been observed that, in this case, introducing a large amount (greater than 10% by weight) of CNTs into the polymer matrix generally has the effect of increasing the viscosity of the compound in the mixing tool, resulting in the screw of the mixer being blocked, requiring the line speed to be reduced and consequently having a negative impact on productivity. Furthermore, stiffening of the composite may result in self-heating which may lead to degradation of the polymer and consequently, in the presence of the CNTs, the formation of a contaminating coating on the walls of the barrels and the screws of the mixer. This results not only in unacceptable contamination of the composite, but also in an increase in the power drawn by the mixer (about 10% over 10 hours of mixing), which then exceeds the power limit of the machine and causes an inadvertent stoppage of said machine. The mixer must then be unblocked and cleaned, thus resulting in a production stoppage.
It has been suggested in application EP 1 995 274 by the Applicant to bring nanotubes into contact with a given plasticizer, in order to form a pre-composite which may then be introduced into a polymer matrix. This solution effectively reduces the viscosity of the mixture, the torque in the extruder and the heating, but has the effect of reducing the amount of polymer.
At the same time, a certain number of studies have shown that it was possible to disperse graphenes in polymer matrices. The article “Graphene-based composite materials”, by Stankovich et al., Nature. vol. 442, p. 282-286 (2006) shows that it is possible to prepare polystyrene-graphene composites by mixing, via a DMF solvent route, polystyrene and graphenes previously grafted by phenyl isocyanate groups. After precipitation via addition of methanol and reduction, the composite is agglomerated by hot-pressing. The electrical percolation threshold is evaluated at 0.1 vol %. For a fraction of 0.5%, the conductivity is 0.1 S/m. However, this method is laborious and expensive.
There is therefore still a need to provide a simple and inexpensive industrial process for continuously preparing composites containing carbon nanotubes, in polymer matrices, without appreciably degrading either the nanotubes or the matrix, and without contaminating the equipment, while at the same time providing an electrical conductivity to the matrix.